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I would use a grid to consider a combination of the supplier/service risk x the potential risk to product safety/quality, legal, security during the activity conducted by the supplier using numbers or traffic lights.
You can gauge your supplier approval based on BRC Guidance: Approval may include a combination of the following:
• membership of a recognised trade association which specialises in the service provided (e.g. pest control)
• historical experience with the supplier
• legal registration (e.g. waste licences)
• third-party certification to a recognised standard (e.g. the BRC Global Standard for Storage and Distribution)
• evidence of training and competence in food safety.
Just like raw material approval you will also need to consider the risk associated with the activity. So let's say contract cleaning of food contact equipment in a high care area. There may be high risk of subsequent pathogen contamination of ready to eat product if the cleaning is not carried out correctly.
If the contracted cleaner is new without any assurances from the approval process you may consider the supplier/service to be high risk and the significance of the hazard (what could go wrong) a high risk. This would mean stringent monitoring procedures such as an ATP swab after cleaning, prior to the start of production.
Based on the supplier/service/risk you may have existing controls which mitigate low risk service risks and you may decide these are only monitored during GMP audits for example.